One of the more useful methods to modify polymers involves blending them with other polymers of different structures and properties. In fact, the subject of polymer blends has been one of the key research and development areas in the field of polymers in the past decade. In a few cases, polymer blend combinations are miscible, i.e., they exhibit thermodynamic miscibility and are mixed at the scale of molecular dimensions. These blends are mechanically compatible.
However, most blends are phase separated and generally exhibit poor mechanical compatibility. Phase separated systems in several cases can exhibit mechanical compatibility where the polymer compositions are similar, e.g., polyolefin blended with other polyolefins, or where interfacial agents are added to improve the compatibility at the interface between the constituents of the polymer blend.
For blends of polyolefins and PVOH, the extremes in polymer compatibility are expectedly observed. Polyolefins are very water insensitive whereas PVOH dissolves in water. Polyolefins are sensitive to hydrophobic compounds, e.g. oils and grease, whereas PVOH is very resistant.
Polymer blends of polyolefins and thermoplastic PVOH are melt processible, however, they exhibit very poor mechanical compatibility. This poor mechanical compatibility is manifested in the mechanical property profile of the blends relative to the properties of the unblended constituents where the blends suffer significantly in elongation at break, tensile strength and toughness. In addition, the blends exhibit non-homogeneity as noted in the injection molded surface and topology of the fractured samples for mechanical testing.
The utility of polyolefin/thermoplastic PVOH blends, if mechanical compatibility could be achieved, can be envisioned for many applications including barrier films, foam, injected molded articles and extrusion applications. The potential utility of polyolefin/thermoplastic PVOH blends has been noted in the patent literature. These patents note the poor mechanical compatibility of polyolefins and thermoplastic PVOH and many note various approaches towards solving this deficiency. Thus, modifications of polyolefin/thermoplastic PVOH blends are necessary in order to obtain useful properties.
U.S. Pat. No. 4,600,746 discloses the compatabilization of PVOH with a functional polymer, i.e., polyolefin or an ethylene-ester copolymer with groups capable of reacting with the hydroxyl of PVOH, e.g. carboxylic acid, anhydride, for barrier polymer blends. Ethylene-acrylic acid, ethylene-methacrylic acid, ionomers, and maleic anhydride grafted polyolefins are disclosed in blends with PVOH as melt processible alloys.
U.S. Pat. No. 4,950,513 discloses a laminate of a polyolefin, a melt blend of a polyamide and a PVOH, and an alkyl carboxyl-substituted polyolefin. The laminate is used as a barrier film composition.
U.S. Pat. No. 4,806,597 discloses a blend of an aromatic polycarbonate, polyolefin, ethylene-vinyl alcohol copolymer and a hydrogenated styrene/butadiene block copolymer modified with unsaturated dicarboxylic acid or anhydride.
U.S. Pat. No. 4,362,844 discloses a biaxially stretched mixture of polypropylene and an ethylene-vinyl alcohol copolymer. Coupling agents proposed to improve the compatibility include maleic anhydride or acrylic acid grafted polypropylene or ethylene-acrylic acid copolymers.
The above references disclose various methods involving modification of olefins to improve compatibility with PVOH. These methods comprise including polar groups in the polyolefin structure or potentially reactive groups, for example, carboxylic acid or anhydride functionality.
Borggreve, R. J. M., et al, "Brittle-Tough Transition In Nylon-Rubber Blends: Effect Of Rubber Concentration And Particle Size", POLYMER, 1987, Vol 28, August, (1489-1496) discloses a one-pass extrusion operation for polyolefin compatibilization with polyamides where the maleic anhydride is grafted to polyolefins utilizing peroxide.